I recently downloaded and installed a 2011-02 64 bit aptosid, which installed without problems, however today I tried to upgrade

apt-get update
apt-get dist-upgrade -d

went well, however when I try to log out of KDE
for the actual update phase I get 'tossed' into
a blank screen with a blinking cursor at the top left and then nothing happens. No messages, no system errors.
I have to power off to continue.

I expected to be able to login as root and continue.

I have tried various ways to exit kde with the same results.

I've tried some of the suggestions offered in the forums without positive results.

Simple thing first, did you try pressing Ctrl+Alt+F1 when you were at the screen with just the blinking cursor? For the rest of this post I'll assume you tried and it made no difference.

What graphics card are you using and what driver?

If it is a radeon do you have the firmware installed (from firmware-linux-nonfree).

What happens if you try to press Ctrl+Alt+F1 while in X (kde), do you see a console login? Can you get back to X with Ctrl+Alt+F7? If you can login via Ctrl+Alt+F1 then you can run "init 3" there and hopefully still have a console where you can complete the dist-upgrade.

You "might" be able to simply tell grub to boot straight into run-level 3 and not start X, or this might leave you at the blank screen also. If it does leave you at a blank screen, you can probably just press the power button to tell it to shut back down cleanly, then try again and try with adding "vga=0 3" and/or "vga=791 3" in grub to boot to runlevel 3 with a couple of different forced framebuffer options. If you try any of the vga codes then reboot without them when you are finished before bothering trying to return to runlevel 5 with X.

If it is a radeon do you have the firmware installed (from firmware-linux-nonfree).

Yes

What happens if you try to press Ctrl+Alt+F1 while in X (kde), do you see a console login? Can you get back to X with Ctrl+Alt+F7? If you can login via Ctrl+Alt+F1 then you can run "init 3" there and hopefully still have a console where you can complete the dist-upgrade.